Conventionally, the following two methods are known as a developing method used in image forming apparatuses using the electrographic method. One is a single-component developing method which uses only toner as developer. The other is a two-component developing method which uses toner and carrier as developer.
Generally, in a single-component developing method, the toner is charged, and a desired thin toner layer is obtained by passing the toner through a regulating portion formed between the toner carrier and a regulating blade pressed against the toner carrier. With this arrangement, the single-component developing method is advantageous for simplification of apparatus, miniaturization, and cost-cutting.
On the other hand, the toner tends to be deteriorated by the strong stress at a regulating portion, and the charge-accepting ability of toner tends to reduce. In addition, the surfaces of the regulating blade and the toner carrier as a charge-providing member are contaminated with toner and additives, and this causes the reduction of the charge-providing ability. Therefore, the charge amount of toner is accordingly lowered to create issues such as fogging. For these reason, the service life of the development device is short.
When two methods are compared, in the two-component developing method, toner is mixed with carrier and is charged by triboelectric charging, thereby causing less stress. Since the area of the carrier is charged, it is not easy for the carrier to be contaminated with toner of external additives. With the result that it is advantageous for a longer service life.
However, in the two-component developing method, when the electrostatic latent image on the image carrier is developed, the surface of the image carrier is rubbed with the magnetic brush formed of a developer. As a result, the two-component developing method has a problem that the magnetic brush marks occur on a developed image. The two-component developing method has another problem that a carrier easily adheres to the image carrier, and the adhered carrier becomes an image defect.
The hybrid developing method has been disclosed (for example, refer to Japanese Laid-Open Patent Publication No. S59-172662) as a developing method that solves the problem of image defect and realizes high image quality at the same level as the one-component developing method while maintaining the advantage of a long lifetime with the two-component developing method using two-component developer. In the hybrid developing method, a two-component developer is supported on the developer carrier, and only toner is supplied to a toner carrier from the two-component developer, whereby the toner is used for development.
However, the hybrid developing method had the following problem.
(1) Reduction in Density at High-Speed Developing
There was a problem that when image formation was carried out at a high speed, the jumping of toner was not enough during the nip time, thereby causing reduction in image density.
The above-mentioned problem is in common in the non-contact single-component developing method. The typical single component developing method is used only in a low speed region since that method gives a strong stress to toner, thereby causing problems of heat generation at a regulating portion and fusion of toner. Therefore, it has not been thought as a big problem. Since the hybrid developing method does not have these restrictions, it can carry out image formation considerably at a high speed. For example, in apparatuses that have a system speed exceed to 500 mm/s, the above-mentioned problem may occur.
(2) Problem of Development Hysteresis (Ghost)
The hybrid development method has a typical problem that post-development residual toner on the toner carrier which was not used for development will appear at the next developing step on an image as development hysteresis (ghost).
The toner to be used for development is supplied in the opposing portion (toner supply area) between the developer carrier for supplying toner to the toner carrier and the toner carrier. However, the collection of the post-development residual toner is also performed in the opposing portion between the toner carrier and the developer carrier. The bias in the supply direction is applied to supply toner, but on the other hand, that bias disturbs the collection of toner, therefore the collecting capability is insufficient. As a result, unevenness of the post-development residual toner will generate a contrast in density in the following developing step.
As a countermeasure to address the density reduction at the time of high-speed development, providing two or more toner carriers is known (see, for example, Japanese Laid-Open Patent Publication No. 2005-37523). This arrangement secures an enough development time for the toner to jump, thereby securing the toner density.
According to the configuration disclosed in Japanese Laid-Open Patent Publication No. 2005-37523, two or more toner carriers cause the toner to jump a plurality of times. Therefore, even when a photoreceptor is rotating at a high speed, a toner image is certainly formed on the photoreceptor, and thereby improving the density reduction related to a higher speed. In the above-mentioned configuration, the respective toner carriers use less toner for development than in the case where a single toner carrier is used. Therefore, on the layer of the post-development residual toner on the toner carriers, there is a smaller difference between a portion where the toner was used for development and a portion where the toner was not used. Therefore, a relatively small ghost will be generated.
In the configuration disclosed in Japanese Laid-Open Patent Publication No. 2005-37523, the ghost is surely improved. However, the study of the inventors of the invention showed that the level of the improvement is not sufficient and the ghost is not controlled sufficiently.
The reason for that problem is that the toner carrier located downstream in the rotating direction of the image carrier is not supplied with sufficient toner. That is because the developer on only one developer carrier supplies toner to a plurality of toner carriers.
In order to cope with the reduction in toner supplying ability, it is possible to compensate the reduction and to control the density reduction by increasing a toner supply bias. However, if the toner supply bias is increased, the electric field to urge toner to the toner carrier, whereby the collection of the post-development residual toner is disturbed from a viewpoint of toner collection. Therefore, the occurrence of ghost is not controlled sufficiently.